At commercial and sport fish farms, interference from marine mammals such as seals and sea lions is a serious problem. The mammals prey on the fish kept in submerged fish pens, resulting in loss to the fish farms. Therefore, it is important to the fish farms to keep the mammals away from their fish pens.
Most marine mammals have excellent hearing and the water in which they live is an efficient medium for transmitting sound. The transmission of underwater sounds as warnings or irritants has been used as a method of repelling marine mammals. A typical acoustic deterrent system for repelling marine mammals from a region of water has circuitry including transmission and control circuitry which drives one or more acoustic projectors located beneath the surface of the region of water. The acoustic projector includes a transmit transducer that delivers periodic bursts of high-frequency pulsed acoustic signals, e.g., between about 7 and 10 kHz, into the water under the fish pens, causing the marine mammals to swim away from the pens.
Sonar devices have widespread use in sport fishing, navigation, scuba diving, as well as any number of other recreational or commercial activities. A sonar system typically includes a sonar unit and an acoustic projector that includes a transmit/receive transducer. The sonar unit includes a display for providing information to the operator. The acoustic projector is mounted under the waterline and is responsible for generating a sound pulse and receiving echoes from objects in the water, from the bottom surface, or both. A typical application of the sonar system is for use as a fish finder.
The sonar unit includes circuitry that produces the sound pulse consisting of several cycles of a sonic signal at a fairly high output power. This pulse is delivered to the transmit/receive transducer via a shielded twisted pair cable. After transmission of the pulse in a transmit mode, the transmit/receive transducer is used to “listen” for echoes in a receive mode. Received echoes produce very small signals, on the order of a few millivolts, which are sent to a receiver circuitry in the sonar unit. In the sonar unit, the received echoes are amplified, filtered, and analyzed.